1. Field of the Invention
The invention concerns the preparation of an optical structure based on optical polymer substrate.
2. Description of Prior Art
Optical polymers are increasingly used to replace glass or for specific new applications. This interest originates first of all because of the facility with which they may be shaped, and also in view of their lightness and possibly their impact resistance. However, they have the disadvantage of showing relatively poor resistance to mechanical wear, as well as against certain agents causing physical and chemical degradation under their conditions of use. Their surface must therefore be protected by means of hard transparent coatings.
In some cases, in addition, there should be provided coatings which are capable of giving optical properties to the finished product, which involve both the specific properties of the film and its interaction with the substrate, for example mechanical behavior at the interface.
The requirements which are the most generally called for by the users concern:
adhesiveness, PA1 abrasion resistance, for example in view of maintaining optical properties, PA1 resistance to cycling and thermal impacts, PA1 inertia towards certain atmospheric or chemical agents, depending on the application under consideration. PA1 --adhesiveness PA1 --thermomechanical behavior PA1 --abrasion resistance PA1 --optical properties PA1 --physicochemical inertia. PA1 a) a plasma treatment for activation of the surface of the substrate, based on a gaseous mixture containing argon; PA1 b) at least one hard layer is deposited by said PECVD technique from silane and oxygen, so as to produce a film on SiO.sub.2, having a thickness of the order of 2 to 5 microns.
Optical specifications may consist of a simple criterion of transparency in the visible range or may include more complex properties.
Some polymers, for example polycarbonate, also undergo an accelerated aging when they are exposed to surrounding ultra-violet radiation (U.V.) under normal conditions of use. When a coating has to be deposited on these polymers, degradation of the interface leads to a still much more rapid deterioration of the product which is accompanied with spectacular delaminations. In such a case, the coating should also filter the U.V. rays.
In a general manner, these optical structures should comply with severe, numerous and diversified accelerated tests, which are more or less specific to the users.
The protective coatings which are normally applied in liquid form are found to offer little flexibility in the modulation of their properties, which are generally imposed by the molecule of the precursor under consideration. It is very difficult to synthesize compounds producing coatings capable of fulfilling all the requirements, and in addition it is naturally impossible to produce other optical properties than simple transparency in the visible range, for example.
Actually, an attempt is being made to replace these liquid solutions with silicon base amorphous inorganic materials, which are deposited from a gaseous phase. Since the use of silane requires certain safety precautions, the first attempts have been made with so-called "plasma polymerization" processes starting from non pyrophoric organosilicon compounds such as hexamethyldisiloxane. The latter is not only a carrier of silicon, but also carries with it the constituent elements of the layer (O, C. H) in a manner which is relatively not easily controllable even by resorting to additional inputs of oxygen (O.sub.2).
To this date, the processes of coating according to the so-called chemical coating process in vapor phase which is improved by plasma ("plasma enhanced chemical vapor deposition" referred to hereinbelow by its usual abbreviation "P.E.C.V.D."), starting from silane on polymers, have not nearly exploited the possibilities which were offered. If materials of good hardness are produced (SiO.sub.2), the achievement of the properties of adhesiveness and heat resistance always require reliance on a primary adhesive applied by liquid means which greatly reduces the economical interest of the process. Moreover, these coatings ensure only a property of protection, without trying to confer optical properties thereto.
The present invention aims at producing, depending on the different requirements of the users, coatings on polymers which simultaneously posses properties of adhesiveness, heat resistance, physical chemical inertia, and possibly a certain number of optical properties. Even if these properties are excluded, it has been found that generally there is no coating material which can alone comply with these various criteria and the technical solution which has been chosen consequently rests on a multilayer composite structure which is rather complex.
The invention therefore aims at providing a group of layer materials having specific properties on the one hand, on the other hand properties of association with one another and with the polymeric substrate, which permits production of a structure responding to all of required specifications.
The processes of coating "PECVD" starting with independent carriers of various elements (SiH.sub.4, O.sub.2, N.sub.2, C.sub.2 H.sub.2 . . . ) permits preparation in a flexible and well controlled manner a large variety of amorphous alloys of silicon. It is often possible, for a given material, to optimize one or more properties, for example the following:
This distinction is quite arbitrary, because, for example, it is obvious that adhesiveness is associated with thermomechanical properties.
Similarly, tests of abrasion are an indication not only of the specific hardness of the layer, but also in a certain measure, of the adhesiveness when abrasive particles of important size have a tendency to stratch in depth, even remove the layer. The physicochemical inertia may also depend on the properties at the materials of the films as well as those of the interfaces.
However, it is possible to attribute a main function to each level of layer.